1. Field of the Invention
The present invention relates generally to a method and an apparatus for filtering a signal, and more particularly, to a method and an apparatus for filtering a Mobile High-definition Link (MHL) signal.
2. Description of the Related Art
In the prior art, due to development of multimedia technology and communication technology, a terminal such as a smart phone or the like can transmit media to a relatively large size multimedia device such as a monitor, a television or the like. Accordingly, a user transmits media stored in the terminal to the relatively large size multimedia device in real time, and thus enjoys the media with the relatively large size multimedia device.
For high quality transmission of the media, a Mobile High-definition Link (MHL) technique has been developed. The MHL technique is a technique enabling compatibility between a micro Universal Serial Bus (USB) and a High Definition Multimedia Interface (HDMI).
As illustrated in FIGS. 1A-1D, the MHL technique can simultaneously transmit a common mode clock (CLK) component of 7.5 MHz and a differential mode data component of 2.2 Gbps by using a pair of differential signals. Accordingly, based on the MHL technique, the number of signals for media transmission is reduced, which corresponds to a greater advantage in allocating pins of a connector.
FIGS. 1A to 1D illustrate examples for describing the MHL technique in the prior art. FIG. 1A illustrates an example of a signal transmission circuit according to the MHL technique, FIG. 1B illustrates an example of an MHL signal according to the MHL technique, FIG. 1C illustrates an example of a common mode clock component included in the MHL signal, and FIG. 1D illustrates an example of a differential mode data component included in the MHL signal.
Since the MHL technique transmits a differential mode data component and a common mode clock component by using a pair of differential signals (hereinafter, referred to as an “MHL signal”), the common mode clock component is distorted when common mode noise flows in, and accordingly, an entire MHL signal is distorted. In particular, shaking of an output screen may be generated or the output screen may be turned off under an RF weak electric field.
In order to solve the above problems, a method of reinforcing shielding of a transmission device (for example, a dongle) is used for transmitting the MHL signal, but the method cannot achieve perfect blocking of the common mode noise.
Meanwhile, in order to satisfy a Radiated Emission (RE) standard, a common mode filter for blocking the common mode noise is applied to a terminal transmitting the MHL signal. However, even in this case, since an RF transmission signal generated in the terminal passes through the common mode filter employed in the terminal and then flows in the common mode clock, the common mode clock component will be distorted. It is because, while the common mode clock component is a signal switched to 400 mV, the RF transmission signal forming the RF weak electric field is a signal corresponding to a maximum of 10 V (33 dBm). Accordingly, even though some RF transmission signal flows in, the common mode clock component will be distorted, thereby resulting in distorting of the entire MHL signal.